WASHINGTON, May 4 - U.S. Democratic presidential front-runner Hillary Clinton took quick aim at presumptive Republican nominee Donald Trump on Wednesday, saying the United States should not take a risk on an unreliable candidate.

"He is a loose cannon, and loose cannons tend to misfire," Clinton said in an interview with CNN, citing Trump stances including an allegation that climate change was a Chinese hoax.

Clinton, a former secretary of state, said Trump would have to offer policy specifics in ahead of the Nov. 8 presidential election and criticized him for his positions on issues including nuclear weapons and abortion.

"He makes these grand statements and grand accusations," Clinton said of the real estate magnate and former reality TV star. "At some point when you're running for president, you actually have to put a little meat on the bones. You've got to tell people what it is you're going to do and how you're going to do it."

Clinton lost the Democratic primary contest in Indiana on Tuesday but still looks set to win her party's nomination, leading her rival, U.S. Senator Bernie Sanders, by more than 800 delegates. Trump's win in Indiana on Tuesday pushed his rivals out of the race and left him as the presumptive nominee.

In a preview of the likely general election battle, Clinton laughed when asked if she was ready to take on Trump, saying: "Oh, please."

"This is to me a classic case of a blustering, bullying guy who has knocked out of the way all the Republicans because they were just dumbfounded," she said.

Clinton said she knew how to run a campaign against Trump and took a swipe at Trump's 16 Republican presidential rivals who started out in the 2016 campaign.

"They didn't know how to deal with him," Clinton said. "They couldn't take him on the issues because they basically agreed with them. And they didn't know how to counterpunch."

Asked how "scorched-earth" the campaign was likely to be, she replied: "You know, he's the one making that decision. ... He's the one who's run the campaign insulting people, demeaning women, degrading people with disabilities, talking about keeping Muslims out of the country, he's the one that's been running that kind of very negative, aggressive, bullying campaign.

"I'm going to keep staying on the kind of campaign I'm running," she said.